1. Field of the Invention
The invention is directed to an improved hydrodewaxing process, whereby the effluent from the hydrodewaxing unit, in addition to having acceptable pour point specifications also exhibits oxidative stability as measured by ASTM D-943.
2. Prior Art
Because lubricating oils for the most part are based on petroleum fractions boiling above about 230.degree. C. (450.degree. F.), the molecular weight of the hydrocarbon constituents is high and these constituents display almost all conceivable structures and structure types. The complexity of the molecular constitution of lubricating oils and its consequences are referred to in "Petroleum Refinery Engineering", by W. L. Nelson, McGraw Hill Book Company, Inc., New York, N.Y., 1958 (Fourth Edition), relevant portions of this text being incorporated herein by reference, for background.
Although the broad principles involved in refining are qualitatively understood, the art is encumbered by quantitative uncertainties which require considerable resort to empiricism in practical refining. Underlying these quantitative uncertainties is the complexity of the molecular constitution of lubricating oils. Accordingly, the art of refining suitable petroleum crude oils to obtain a variety of lubricating oils which function effectively in diverse environments has become highly developed and complex.
In general, the basic notion in lubricant refining is that a suitable crude oil, as shown by experience or by assay, contains a quantity of lubricant stock having a predetermined set of properties such as, for example, appropriate viscosity, oxidation stability, and maintenance of fluidity at low temperatures. The process of refining to isolate that lubricant stock consists of a set of subtractive unit operations which removes the unwanted components. The most important of these unit operations include distillation, solvent refining, and dewaxing, which basically are physical separation processes in the sense that if all the separated functions were recombined, one would constitute the crude oil.
Lubricating oil stocks generally have an unacceptably high pour point and thus require dewaxing. Solvent dewaxing is a well-known and effective process but expensive. More recently catalytic methods for dewaxing have been proposed. U.S. Pat. No. Re. 28,398 describes a catalytic dewaxing process wherein a particular crystalline zeolite is used. U.S. Pat. Nos. 4,283,271, 4,283,272 and 4,414,097 describe processes for producing dewaxed lubricating oil base including hydrocracking a hydrocarbon feedstock, catalytically dewaxing the hydrocrackate and hydrotreating the dewaxed hydrocrackate. These latter patents, recognized as advances in the art, describe use of catalyst compositions comprising zeolite ZSM-5, ZSM-11 and ZSM-23 for the dewaxing phase. To obtain lubricants and specialty oils with outstanding resistance to oxidation, it is often necessary to hydrotreat the oil after catalytic dewaxing, as illustrated by U.S. Pat. No. 4,137,148. The foregoing patents indicate the state of the dewaxing art and are incorporated herein by reference as background.
As indicated above, the prior art processes include a hydrotreating stage subsequent to hydrodewaxing. Although hydrodewaxing produces effluents of acceptable pour point and viscosity index specification, the same effluents may be unstable to air and/or to light. The object of the hydrotreating step, after hydrodewaxing, is to remove by hydrogenation those components of the dewaxed effluent which were unstable to oxidation conditions, unstable to air and light.
The hydrotreating operation can include passing the dewaxed effluent to a hydrotreater unit. The hydrotreater unit contains a hydrotreating catalyst in a hydrotreating zone at stabilizing conditions. Examples of hydrotreating catalysts include, without limitation, one or more metals from Group VIII (e.g., cobalt and nickel) and Group VI (e.g., molybdenum and tungsten) of the Periodic Table of Elements supported by an inorganic oxide such as, for example, alumina or silica-alumina. Conditions in the hydrotreater unit are summarized in Table I.
TABLE I ______________________________________ Hydrotreating Conditions ______________________________________ Pressure, broad, psig 400-3000 Pressure, preferred, psig 400-2500 Temperature, broad, .degree.C. 176-371 Temperature, preferred, .degree.C. 204-316 LHSV,* broad 0.1-10 LHSV, preferred 0.2-3 H.sub.2 gas, SCF/bbl, broad 500-20,000 H.sub.2 gas, SCF/bbl, preferred 500-3000 ______________________________________
It is an object of the invention to eliminate the necessity of the additional hydrotreating operation in producing hydrodewaxed effluents which are stable to oxidation by air and/or by light. It is thus an object of the invention to provide a hydrodewaxing operation which results in a hydrodewaxed effluent which is stable to air and to light.